1. Field of Invention
The present invention relates to improved catalysis in coal liquefaction processes.
2. Prior Art
The extensive coal reserves of the United States provide a potentially vast source of petrochemical energy, provided their conversion to a usable form can be economically accomplished. A substantial amount of government funded research has been directed toward commercial processes for both gasification and liquefaction of coal; however, the high costs of such processes still remain a primary deterrent to the utilization of this abundant source of energy.
With respect to the field of coal liquefaction, numerous techniques have been developed, including (1) gasification to CO and H.sub.2 followed by synthesis to liquid hydrocarbons, (2) carbonization, (3) hydrogenation of coal-oil slurry, and (4) dry coal hydrogenation. A major difficulty with all these processes is the minimizing of energy consumed to effect the conversion process. To reduce reaction time and accomplish the reaction at lower temperature, a catalyst is customarily involved at some point in most liquefaction methods.
The selection of a particular catalyst depends upon the nature of the reaction conditions, viewed in relation to previous catalyst experience derived from a trial and error approach. In the absence of conclusive theoretical basis, the current problem solving process associated with catalyst selection has developed into a unique art which continues to rely in part on a trial and error process. This lack of understanding has resulted in the random identification of specific compounds which have experimentally demonstrated some utility as hydrogenating catalysts without sufficient theoretical understanding to maximize their efficiency. A partial list of such identified catalysts is disclosed in Anderson, Wood, and Wiser, "Clean Liquid Energy from Coal", Society of Mining Engineers of AIME, Preprint No. 75-F-318 (1975).
Heretofore the choice of catalyst and method of application in coal hydrogenation has not been based on any common unique physical properties associated with the identified catalysts. Because of the lack of such common properties, a class definition has been limited to those compounds which provide the chemical result of effective hydrogenation under specified reaction conditions. Such a definition is of little assistance in improving current catalysis methods and searching for new materials.
The techniques of coal liquefaction catalysis have been varied. In 1968 the Office of Coal Research completed an extensive study in which molten ZnCl.sub.2 in large concentrations was investigated as a catalysis environment. United States Department of Interior, OCR Research and Development Report No. 39, Vol. III, Book 1, "Research on Zinc Chloride Catalyst for Converting Coal to Gasoline." Unfortunately, the process did not prove to be economically feasible. The high energy cost of maintaining the ZnCl.sub.2 in melt form and the catalyst loss associated with the process involve costs beyond that which the market would endure.
In a series of U.S. patents relating to dry coal processing (U.S. Pat. Nos. 3,152,063; 3,823,084; 3,926,775; and 3,944,480) Schroeder discusses the use of catalysts selected from those known in the art, namely tungsten or molybdenum oxides or sulfides, tin or iron group metals such as iron, nickel, cobalt and their compounds. The suggested means of applying the catalyst in the first patents was by impregnation of the catalyst on the surface of the coal particles by means of a slurry of catalyst in solution. The latter patents disclose the use of catalyst bed reactions wherein the coal particles are carried through the catalyst bed by means of a stream of hydrogen gas.
Attempts to apply the teachings of these patents and other prior art related to coal hydrogenation-liquefaction have failed to meet the requirements for an economical system. Utilizing small tube diameter reactor systems, the present inventors evaluated the respective catalysts shown below under reaction conditions of 650.degree. C. and H.sub.2 pressure of 1750 psi and flow rate of 3.5 standard cubic feet per minute:
______________________________________ Coal Catalyst Conversion % ______________________________________ ZnBr.sub.2 58.5 ZnI.sub.2 46.3 ZnCl.sub.2 41.1 SnCl.sub.2 . 2H.sub.2 O 40.5 SnCl.sub.4 . 5H.sub.2 O 25.6 LiI 16.6 CrCl.sub.2 12.8 Pb(C.sub.2 H.sub.3 O.sub.2).sub.2 . 3H.sub.2 O 11.7 NH.sub.4 Cl 11.0 CdCl.sub.2 . 5H.sub.2 O 7.9 Sn (powder) 7.9 CnCl.sub.2 . 2H.sub.2 O 7.6 FeCl.sub.3 . 6H.sub.2 O 7.2 Zn (powder) 7.0 ZnSO.sub.4 . 7H.sub.2 O 5.4 (NH.sub.4).sub.6 Mo.sub.7 O.sub.24 . 4H.sub.2 O 5.4 FeCl.sub.2 3.3 CaCl.sub.2 . H.sub.2 O no reaction Na.sub.2 CO.sub.3 . H.sub.2 O no reaction ______________________________________
As a result of this evaluation it was discovered that most of the catalysts suggested in the Schroeder patents failed to produce sufficient conversion yield to offset the costs of catalyst replenishment and recovery. Of the remaining catalysts which reflect the higher percents of conversion, the requirements of using in excess of 5 to 10% catalyst lead to increased cost due to their expensive character and difficulties in catalyst recovery. In addition, the application of prior art teachings relating to catalyst impregnation by means of a slurry coating followed by drying, would cause increased energy losses. Essentially, the prior art remains encumbered by the recurring economic obstacle of a noncompetitive position with respect to crude oil imports, suggesting the need for a new approach to the catalysis step in coal liquefaction.